1. Field of Invention
The present invention primarily pertains to a light-emitting semiconductor device pulse drive method and pulse drive circuit permitting adjustment of luminance and/or adjustment of chromaticity, and capable of being used in apparatuses such as those for liquid crystal backlighting and/or illumination apparatuses employing light emitting diode(s) as white light source(s).
2. Conventional Art
As conventional art for carrying out adjustment of luminance in light-emitting semiconductor devices incorporating light emitting diodes or the like, a color display apparatus in which a light source containing a plurality of light-emitting elements (e.g., light emitting diodes) of different colors that are switched and driven in fixed order and in which control of light intensity is carried out using optical sensor(s) to detect luminance level(s) thereof has, for example, been proposed at Japanese Patent Application Publication Kokai No. H10-49074(1998) (hereinafter “Patent Reference No. 1”).
In the context of a color display apparatus carrying out multicolor display and having a light source unit containing a plurality of light-emitting elements of respectively different colors emitting light of different wavelengths and respectively permitting independent adjustment of luminance; a light source drive circuit switching in fixed order the plurality of light-emitting elements of different colors of the light source unit and sequentially supplying respective drive signals thereto; a shutter unit comprising electro-optic shutter elements constituting a plurality of pixels and controlling transmission/blocking of light emitted by color light-emitting elements of the light source unit; and a shutter control circuit supplying, to said shutter unit, color signals corresponding to emission of light from the plurality of color light-emitting elements of the light source unit in synchronous fashion with respect to times at which such emission of light occurs, this color display apparatus comprises optical sensor(s) detecting luminance levels of the respective color light sources; and a light intensity control circuit supplying light intensity control signals to the light source drive circuit in accordance with values detected by said sensor(s); wherein adjustment of respective light source luminance levels is carried out so as to maintain constant balance among luminances of respective color light sources for desired color. As many optical sensors may be provided as the number of emitted colors, or only a single optical sensor may be employed to accommodate same. As method for adjusting luminance, light-emitting element(s) of specific color(s) is/are made to light, optical output therefrom is converted into electricity and is smoothed by means of an integrating circuit, and drive current is controlled through use of the signal obtained.
At this Patent Reference No. 1, however, while a method is described in which a single optical sensor is used in shared fashion for respective emitted colors, there is neither any consideration made nor any specific disclosure with respect to a constitution such as might be suited to obtaining detection output that takes into account the light that is emitted from all of the light-emitting elements in situations, such as with illumination sources, where a multiplicity of light-emitting elements are arranged in distributed fashion.
An illumination apparatus in which the region at which the light-emitting elements are mounted is covered with a transparent resin layer and this transparent resin layer is capable of appropriately guiding, to optical sensor(s), light from a multiplicity of light-emitting elements arranged in distributed fashion has, for example, therefore been proposed at Japanese Patent Application Publication Kokai No. 2002-344031 (hereinafter “Patent Reference No. 2”).
This illuminating apparatus comprises a plurality of light emitting diodes arranged in distributed fashion in at least two dimensions; a transparent resin layer integrally covering the plurality of light emitting diodes; an optical detector unit employing optical detector element(s) arranged within and/or on the surface of the transparent resin layer and/or in the vicinity thereof to detect intensity or intensities of light emitted by the light emitting diodes; and a power supply circuit unit controlling driving of the light emitting diodes based on output produced as a result of detection by the optical detector unit; wherein the number of the optical detector element(s) is smaller than the number of the light emitting diodes; and the optical detector element(s) detect intensity or intensities of light emitted by the light emitting diodes after same has propagated thereto through the transparent resin layer.
Furthermore, another example of the conventional art is a method in which red, green, and blue light-emitting elements are sequentially made to light in staggered fashion, there being a fixed time interval between the times at which lighting of each is initiated, and optical output of each color alone is determined by arithmetic processing from the difference in optical output measured at each such occasion. To wit, the red light-emitting element is first made to light and the optical output thereof is measured. The green light-emitting element is next made to light and the optical output produced as a result of combination of the red and green light-emitting elements is measured, and the red output component which was previously measured is subtracted therefrom to obtain the optical output of the green light-emitting element. The blue light-emitting element is, moreover, made to light and the optical output produced as a result of combination of light-emitting elements of all three colors is measured, and the optical output component produced as a result of combination of the red and green light-emitting elements is subtracted therefrom to obtain the optical output of the blue light-emitting element. There is also a method for driving a light-emitting semiconductor device in pulsed fashion wherein arithmetic processing is carried out on optical outputs for respective colors obtained in such fashion and drive current values such as will produce optimum luminance and chromaticity are determined. However, where driving of light-emitting elements, and optical sensor(s) or optical sensor electrical signal conversion circuit(s), are merely sequentially switched in turn by emitted color, such as is the case in the conventional art disclosed at the foregoing Patent Reference No. 1, no consideration is made with respect to circuit delay time and light-emitting element rise and fall times and so forth. This being the case, times during which light-emitting elements of different emitted color are actually lit can partially overlap, and this can cause error in the detection of optical output and decrease in the precision with which luminance is adjusted. Furthermore, in the conventional art disclosed at Patent Reference No. 1, control of electric current is such that electric current is reduced relative to a prescribed maximum electric current, comparison with the signal obtained after integration and smoothing of optical output permitting driving of electric current to be carried out in stable fashion at a prescribed level. However, in order to cause lighting to occur with good precision by this method, for integration and smoothing it is necessary to constantly synchronize lit time start and end times with electric current control times. That is, integration time is necessary for integration and smoothing, and because there is occurrence of temporal delays, processing time for same must also be taken into consideration, necessitating extremely complex circuit structure. Furthermore, because it is assumed that there is sequential lighting in turn by emitted color, lighting of light-emitting elements is limited to one color at a time. Because light intensity is insufficient as compared with the situation where light-emitting elements of all emitted colors are lit simultaneously, this is not especially suited for achieving the high luminance such as is sought for illumination apparatuses and the like.
In the third and fourth embodiments at the foregoing Patent Reference No. 2, timing charts are disclosed for causing respective red, green, and blue light emitting diodes to sequentially emit light, in which pulsed voltages synchronized with the interval during which a prescribed clock signal goes high are sequentially applied to the respective light emitting diodes. In such case, while it is true that all of the respective light emitting diodes will consequently be unlit throughout the interval during which the clock signal goes low, it is however not the case that the length of this unlit time will have been set in optimal fashion after taking into consideration the circuit delay time and light-emitting element rise and fall times and so forth. Where the unlit time is longer than necessary, this can represent a constraint when attempting to increase duty cycle during pulse drive and can limit ability to achieve increased luminance in illumination apparatuses and the like.
With respect to that which was indicated above as another example of the conventional art, it is in a sense true that simultaneous lighting of all colors is possible at times other than when measurement is taking place and that there should be no constraint with respect to achievement of high luminance. However, because it is necessary to store measured optical output values and to carry out subtraction and other such arithmetic processing, it would be difficult to construct this through employment of only electrical circuitry of relatively small scale. An arithmetic processor capable of accommodating on-chip software or the like would be necessary, making for a complicated and elaborate illumination apparatus.